Handover control method, cell reselection method and mobile station

ABSTRACT

A handover control method comprises: a step in which a mobile station measures radio qualities in a plurality of different frequency cells at predetermined measurement intervals; and a step in which the mobile station, when determining that the radio quality in a specific frequency cell satisfies a report condition, starts up a timer for measuring the time during which the radio quality in the specific frequency cell continues to satisfy the report condition and, when the timer has expired, transmits to a radio base station a measurement report including the radio quality in the specific frequency cell. The mobile station, after determining that the radio quality in a second frequency cell satisfies the report condition and starting up a second timer, stops the second timer and starts up a first timer when determining that the radio quality in a first frequency cell having a higher priority than that of the second frequency cell satisfies the report condition.

TECHNICAL FIELD

The present invention relates to a handover control method forperforming a handover control on the mobile station by a radio basestation on the basis of a measurement report transmitted from the mobilestation, and to a mobile station.

The present invention also relates to a cell reselection method forreselecting a cell for performing a standby on the basis of radioqualities in multiple different frequency cells, the radio qualitiesbeing measured by the mobile station at predetermined measurementintervals, and to a mobile station.

BACKGROUND ART

In an LTE (Long Term Evolution) mobile communication system, whendetermining that a radio quality in a specific frequency cell satisfiesa reselection criterion, a mobile station UE in an Idle state activatesa timer Treselection configured to measure a period during which theradio quality in the specific frequency cell continues to satisfy thereselection criterion.

Thereafter, when the timer Treselection expires, the mobile station UEin the Idle state reselects the specific frequency cell as a cell forthe mobile station UE to newly stand by.

In contrast, when detecting that the radio quality in the specificfrequency cell does not satisfy the reselection criterion any more, themobile station UE in the Idle state resets the timer Treselection.

Meanwhile, in the LTE mobile communication system, when determining thata radio quality in a specific frequency cell satisfies a reportingcriterion, a mobile station UE in an RRC_Connected state activates atimer Ttrigger configured to measure a period during which the radioquality in the specific frequency cell continues to satisfy thereporting criterion.

Thereafter, when the timer Ttrigger expires, the mobile station UE inthe RRC_Connected state transmits a “Measurement Report” including theradio quality in the specific frequency cell to a radio base stationeNB.

In contrast, when detecting that the radio quality in the specificfrequency cell does not satisfy the reporting criterion any more, themobile station UE in the RRC_Connected state resets the timer Trigger.

Meanwhile, in the LTE mobile communication system, when the mobilestation UE in the Idle state can standby in multiple different frequencycells, the radio base station eNB can assign a priority for eachfrequency cell for the mobile station UE in the Idle state, the prioritybeing used in cell reselection.

However, the conventional LTE mobile communication system has thefollowing problem. Specifically, when the timer Treselection isactivated for multiple frequency cells assigned different priorities, itis not clear how the mobile station UE should perform the cellreselection.

In the conventional LTE mobile communication system, it is not specifiedthat the radio base station eNB can assign priorities to be used intransmitting a “Measurement Report.” If the specification is provided,it is assumed that the same problem as the problem in the aforementionedcell reselection occurs.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the aforementionedproblems. An object of the present invention is to provide a handovercontrol method, a cell reselection method, and a mobile station forwhich a specification is provided to clarify, when a timer Treselectionor a timer Ttrigger is activated, how the mobile station UE shouldperform a cell reselection or transmit a “Measurement Report” inmultiple frequency cells assigned different priorities.

A first aspect of the present invention is summarized as a handovercontrol method by which a radio base station performs a handover controlon a mobile station on a basis of a measurement report transmitted fromthe mobile station, the method comprise measuring radio qualities in aplurality of different frequency cells by the mobile station in apredetermined measurement period when the mobile station determines thata radio quality in a specific frequency cell satisfies a reportingcriterion, activating a timer by the mobile station, the timer beingconfigured to measure a period during which the radio quality in thespecific frequency cell continues to satisfy the reporting criterion andwhen the timer expires, transmitting the measurement report to the radiobase station by the mobile station, the measurement report including theradio quality in the specific frequency cell when the mobile stationdetermines that a radio quality in a first frequency cell having ahigher priority than a second frequency cell satisfies the reportingcriterion after determining that a radio quality in the second frequencycell satisfies the reporting criterion and activating a second timer,the mobile station stops the second timer and actives a first timer.

In the first aspect, wherein when stopping the second timer andactivating the first timer, the mobile station stops measuring the radioquality in the second frequency cell and starts measuring the radioquality in the first frequency cell, in a period for measuring the radioquality in the second frequency cell.

In the first aspect, wherein after transmitting the measurement reportincluding the radio quality in the first frequency cell, the mobilestation resumes measuring the radio quality in the second frequency cellin the period for measuring the radio quality in the second frequencycell.

In the first aspect, wherein even after transmitting the measurementreport including the radio quality in the first frequency cell, themobile station continues measuring the radio quality in the firstfrequency cell in the period for measuring the radio quality in thesecond frequency cell, and transmits the measurement report includingthe radio quality in the first frequency cell at predeterminedintervals.

In the first aspect, wherein when transmitting the measurement reportincluding the radio quality in the second frequency cell atpredetermined intervals, the mobile station does not stop measuring theradio quality in the second frequency cell in the period for measuringthe radio quality in the second frequency cell.

A second aspect of the present invention is summarized as a cellreselection method by which a mobile station reselects a cell forperforming a standby on a basis of radio qualities in a plurality ofdifferent frequency cells, the radio qualities being measured in apredetermined measurement period, the method comprise when the mobilestation determines that a radio quality in a specific frequency cellsatisfies a reselection criterion, activating a timer by the mobilestation, the timer being configured to measure a period during which theradio quality in the specific frequency cell continues to satisfy thereselection criterion and starting the standby in the specific frequencycell by the mobile station when the timer expires when the mobilestation determines that a radio quality in a first frequency cell havinga higher priority than a second frequency cell satisfies the reselectioncriterion after determining that a radio quality in the second frequencycell satisfies the reselection criterion and activating a second timer,the mobile station stops the second timer and actives a first timer.

In the second aspect, wherein when stopping the second timer andactivating the first timer, the mobile station stops measuring the radioquality in the second frequency cell and starts measuring the radioquality in the first frequency cell, in a period for measuring the radioquality in the second frequency cell.

A third aspect of the present invention is summarized as a mobilestation used in a mobile communication system configured such that aradio base station performs a handover control on the mobile station ona basis of a measurement report transmitted from the mobile station, themobile station comprise a measurement unit configured to measure radioqualities in a plurality of different-frequency cells in a predeterminedmeasurement period and a measurement report transmitter unit configuredto activate a timer when the measurement report transmitter unitdetermines that a radio quality in a specific frequency cell satisfies areporting criterion, and to transmit the measurement report to the radiobase station when the timer expires, the timer being configured tomeasure a period during which the radio quality in the specificfrequency cell continues to satisfy the reporting criterion, themeasurement report including the radio quality in the specific frequencycell when the measurement report transmitter unit determines that aradio quality in a first frequency cell having a higher priority than asecond frequency cell satisfies the reporting criterion afterdetermining that a radio quality in the second frequency cell satisfiesthe reporting criterion and activating a second timer, the measurementreport transmitter unit stops the second timer and actives a firsttimer.

In the third aspect, wherein when stopping the second timer andactivating the first timer, the measurement report transmitter unitstops measuring the radio quality in the second frequency cell andstarts measuring the radio quality in the first frequency cell, in aperiod for measuring the radio quality in the second frequency cell.

In the third aspect, wherein after the measurement report transmitterunit transmits the measurement report including the radio quality in thefirst frequency cell, the measurement unit resumes measuring the radioquality in the second frequency cell in the period for measuring theradio quality in the second frequency cell.

In the third aspect, wherein even after the measurement reporttransmitter unit transmits the measurement report including the radioquality in the first frequency cell, the measurement unit continuesmeasuring the radio quality in the first frequency cell in the periodfor measuring the radio quality in the second frequency cell, and themeasurement report transmitter unit transmits the measurement reportincluding the radio quality in the first frequency cell at predeterminedintervals.

In the third aspect, wherein when the measurement report transmitterunit transmits the measurement report including the radio quality in thesecond frequency cell at predetermined intervals, the measurement unitdoes not stop measuring the radio quality in the second frequency cellin the period for measuring the radio quality in the second frequencycell.

A fourth aspect of the present invention is summarized as a mobilestation configured to reselect a cell for performing a standby on abasis of radio qualities in a plurality of different frequency cells,the radio qualities being measured in a predetermined measurementperiod, the mobile station comprise a measurement unit configured tomeasure the radio qualities in the plurality of different-frequencycells in the predetermined measurement period and a cell reselector unitconfigured to activate a timer when the cell reselector unit determinesthat a radio quality in a specific frequency cell satisfies areselection criterion, and to start the standby in the specificfrequency cell when the timer expires, the timer being configured tomeasure a period during which the radio quality in the specificfrequency cell continues to satisfy the reselection criterion when thecell reselector unit determines that a radio quality in a firstfrequency cell having a higher priority than a second frequency cellsatisfies the reselection criterion after determining that a radioquality in the second frequency cell satisfies the reselection criterionand activating a second timer, the cell reselector unit stops the secondtimer and actives a first timer.

In the fourth aspect, wherein when the cell reselector unit stops thesecond timer and activates the first timer, the measurement unit stopsmeasuring the radio quality in the second frequency cell and startsmeasuring the radio quality in the first frequency cell, in a period formeasuring the radio quality in the second frequency cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall configuration diagram of a mobile communicationsystem according to a first embodiment of the present invention.

FIG. 2 is a functional block diagram of a mobile station according tothe first embodiment of the present invention.

FIG. 3 is a diagram for explaining a method of measuring radio qualitiesin different frequency cells by the mobile station according to thefirst embodiment of the present invention and a method of transmitting a“Measurement Report” by the mobile station.

FIG. 4 is a diagram for explaining the method of measuring the radioqualities in the different frequency cells by the mobile stationaccording to the first embodiment of the present invention and themethod of transmitting a “Measurement Report” by the mobile station.

FIG. 5 is a is a diagram for explaining a method of measuring radioqualities in different frequency cells by a mobile station according toa second embodiment of the present invention and a method oftransmitting a “Measurement Report” by the mobile station.

FIG. 6 is a diagram for explaining the method of measuring the radioqualities in the different frequency cells by the mobile stationaccording to the second embodiment of the present invention and themethod of transmitting a “Measurement Report” by the mobile station.

BEST MODES FOR CARRYING OUT THE INVENTION

(Mobile Communication System. According to First Embodiment of thePresent Invention)

A description is given of a mobile communication system according to afirst embodiment of the present invention by referring to FIG. 1 to FIG.4. In this embodiment, an LTE mobile communication system is describedas an example of a mobile communication system. However, the presentinvention is not limited to the example.

In the mobile communication system according to this embodiment, a radiobase station eNB is configured to perform a handover control on a mobilestation UE on the basis of a “Measurement Report” transmitted from themobile station UE.

In addition, in the mobile communication system according to thisembodiment, the mobile station UE is configured to reselect a cell forperforming a standby on the basis of radio qualities in multipledifferent frequency cells, the qualities being measured at predeterminedmeasurement intervals (in measurement gaps).

In this embodiment, a frequency cell F0, a frequency cell F1, afrequency cell F2 and a frequency cell F3 are arranged as shown inFIG. 1. Both a priority taken into consideration in reselecting a celland a priority taken into consideration in transmitting a “MeasurementReport” are higher in the order of the frequency cell F1, the frequencycell F0, the frequency cell F2 and the frequency cell F3.

As shown in FIG. 2, the mobile station UE according to this embodimentincludes a priority manager 11, a measurement unit 12, a measurementreport transmitter 13 and a cell reselector 14.

The priority manager 11 is configured to manage, for each frequencycell, the priority taken into consideration in reselecting a cell andthe priority taken into consideration in transmitting a “MeasurementReport” as described above.

The measurement unit 12 is configured to measure radio qualities inmultiple different frequency cells at predetermined intervals (inmeasurement gaps).

In this embodiment, since a serving cell of the mobile station UE is thefrequency cell F0 as shown in FIG. 1, the measurement unit 12 isconfigured to measure radio qualities in the frequency cell F1, thefrequency cell F2 and the frequency cell F3 in each measurement gap,except for a case to be described later.

The measurement report transmitter 13 is configured to activate a timerTtrigger when the measurement report transmitter 13 determines that aradio quality in a specific frequency cell satisfies a reportingcriterion. The timer Ttrigger is configured to measure a period duringwhich the radio quality in the specific frequency cell continues tosatisfy the reporting criterion. When the timer Ttrigger expires, themeasurement report transmitter 13 transmits a “Measurement Report”including the radio quality in the specific frequency cell to the radiobase station eNB.

The cell reselector 14 is configured to activate a timer Treselectionwhen the cell reselector 14 determines that a radio quality in aspecific frequency cell satisfies a reselection criterion. The timerTreselection is configured to measure a period during which the radioquality in the specific frequency cell continues to satisfy thereselection criterion. When the timer Treselection expires, the cellreselector 14 starts a standby in the specific frequency cell.

By referring to FIG. 3 and FIG. 4, a detailed description is hereinbelowgiven of a method of measuring the radio qualities in the differentfrequency cells by the mobile station UE according to this embodimentand a method of transmitting a “Measurement Report” by the mobilestation UE.

FIG. 3 shows temporal changes of radio qualities (RSRP: Reference SignalReceived Power) in the frequency cells F0 to F3.

For example, the following five kinds of events A1 to A5 are assumableas the aforementioned reporting criterion.

(Event A1) Serving>Thresh

(Event A2) Serving<Thresh

(Event A3) Neighbour>Serving+Offset

(Event A4) Neighbour>Thresh

(Event A5) Serving<Thresh1 & Neighbour>Thresh2

“Serving” is a radio quality (RSRP) in a serving cell. “Neighbour” is aradio quality (RSRP) in a neighbouring cell of the serving cell.“Thresh,” “Thresh1,” and “Thresh2” are predetermined thresholds.“Offset” is a parameter for adjustment.

For example, Event A4 is used as a reporting criterion for a case wherethe priority of a neighbouring cell as a measurement target is higherthan the priority of the serving cell. Event A5 is used as a reportingcriterion for a case where the priority of the neighbouring cell as ameasurement target is lower than the priority of the serving cell.

In addition, the following two kinds of transition conditions areassumable as the reselection criterion.

(1) Neighbour>Threshx,high

(2) Serving<Threshserving & Neighbour>Threshx,high

The criterion (1) is used as a reselection criterion for a case wherethe priority of a neighbouring cell as a measurement target is higherthan the priority of the serving cell. The criterion (2) is used as areselection criterion for a case where the priority of the neighbouringcell as a measurement target is lower than or equal to the priority ofthe serving cell.

In Section A, the measurement unit 12 repeatedly measures the radioqualities in each measurement gap, in the order of the frequency cellF1, the frequency cell F2 and the frequency cell F3.

At a time point t1, the measurement report transmitter 13 determinesthat the radio quality in the frequency cell F2 (second frequency cell)satisfies any of the reporting criteria (for example, Event A5), andactivates a timer T2 (second timer corresponding to the aforementionedTtrigger) for the frequency cell F2.

In Section B, the measurement unit 12 stops measuring the radio qualityin the frequency cell F3 having the lower priority than the priority ofthe frequency cell F2, in the measurement gap, and starts measuring theradio quality in the frequency cell F2 in a period assigned formeasuring of the radio quality in the frequency cell F3.

To put it differently, in Section B, the measurement unit 12 repeatedlymeasures the radio qualities in the order of the frequency cell F1, thefrequency cell F2 and the frequency cell F2, in the measurement gap.

Subsequently, at a time point t2, when determining that the radioquality in the frequency cell F1 (first frequency cell) having thehigher priority than the frequency cell F2 (second frequency cell)satisfies any of the reporting criteria (for example, Event A4), themeasurement report transmitter 13 stops the timer T2 (second timer) forthe frequency cell F2 and activates a timer T1 (first timercorresponding to the aforementioned Ttrigger) for the frequency cell F1.

In Section C, the measurement unit 12 stops measuring the radioqualities in the frequency cells F2 and F3 each having the lowerpriority than the priority of the frequency cell F1, in the measurementgap, and starts measuring the radio quality in the frequency cell F1 inperiods respectively for measuring of the radio qualities in thefrequency cells F2 and F3.

To put it differently, in Section C, the measurement unit 12 measuresthe radio quality in the frequency cell F1 in the entire period of themeasurement gap.

Subsequently, at a time point t3, if the timer T1 for the frequency cellF1 expires without being reset, the measurement report transmitter 13transmits a “Measurement Report” including the radio quality in thefrequency cell F1 to the radio base station eNB.

Note that when “Event Triggered Periodical Reporting” is applied to amethod of transmitting a “Measurement Report,” the measurement reporttransmitter 13 transmits the “Measurement Report” N times at maximum atpredetermined reporting intervals (for example, every R1 second) evenafter the time point t3.

In such case, in Section D, the measurement unit 12 continues measuringthe radio quality in the frequency cell F1 in the entire period of themeasurement gap.

In contrast, when “Event Triggered Reporting” is applied to the methodof transmitting a “Measurement Report,” the measurement reporttransmitter 13 transmits a “Measurement Report” at the time point t3,and thereafter in Section D, the measurement unit 12 resumes measuringthe radio qualities in the frequency cells F2 and F3 in periods formeasuring the radio qualities in the frequency cells F2 and F3.

Note that, in aforementioned Sections B and C, the measurement unit 12does not have to stop measuring the radio qualities in the frequencycells F2 and F3.

Similarly, by referring to FIG. 3 and FIG. 4, a description is given ofthe method of measuring the radio qualities in the different frequencycells by the mobile station UE according to the present embodiment and amethod of reselecting a cell by the mobile station UE.

In Section A, the measurement unit 12 repeatedly measures the radioqualities in the order of the frequency cell F1, the frequency cell F2and the frequency cell F2, in the measurement gap.

At the time point t1, the cell reselector 14 determines that the radioquality in the frequency cell F2 (second frequency cell) satisfies thereselection criterion (2), and activates the timer T2 (second timercorresponding to the aforementioned Treselection) for the frequency cellF2.

In Section B, the measurement unit 12 stops measuring the radio qualityin the frequency cell F3 having the lower priority than the priority ofthe frequency cell F2, in the measurement gap, and starts measuring theradio quality in the frequency cell F2 in the period for measuring ofthe radio quality in the frequency cell F3.

To put it differently, in Section B, the measurement unit 12 repeatedlymeasures the radio qualities in the order of the frequency cell F1, thefrequency cell F2 and the frequency cell F2, in the measurement gap.

Subsequently, at the time point t2, when determining that the radioquality in the frequency cell F1 (first frequency cell) having thehigher priority than the frequency cell F2 (second frequency cell)satisfies the reselection criterion (1), the cell reselector 14 stopsthe timer T2 (second timer) for the frequency cell F2 and activates thetimer T1 (first timer corresponding to the aforementioned Treselection)for the frequency cell F1.

In Section C, the measurement unit 12 stops measuring the radioqualities in the frequency cells F2 and F3 each having the lowerpriority than the priority of the frequency cell F1, in the measurementgap, and starts measuring the radio quality in the frequency cell F1 inperiods respectively for measuring of the radio qualities in thefrequency cells F2 and F3.

To put it differently, in Section C, the measurement unit 12 measuresthe radio quality in the frequency cell F1 in the entire period of themeasurement gap.

Subsequently, at the time point t3, if the timer T1 for the frequencycell F1 expires without being reset, the cell reselector 14 starts astandby in the frequency cell F1.

Note that, in the aforementioned Sections B and C, the measurement unit12 does not have to stop measuring the radio qualities in the frequencycells F2 and F3.

The mobile communication system according to the first embodiment of thepresent invention is configured as follows. Even in a case where thetimer T2 for the frequency cell F2 is active, the timer T2 for thefrequency cell F2 is stopped on condition that the timer T1 for thefrequency cell F1 having the higher priority is activated. Accordingly,the mobile station UE can perform a handover or a cell reselection to afrequency cell having a higher priority.

(Mobile Communication System According to Second Embodiment of thePresent Invention)

A mobile communication system according to a second embodiment of thepresent invention will be described by referring to FIG. 5 and FIG. 6.Hereinbelow, the description is given of the mobile communication systemaccording to the second embodiment of the present invention, focused ona difference from the mobile communication system according to the firstembodiment of the present invention.

By referring to FIG. 5 and FIG. 6, a detailed description is given of amethod of measuring radio qualities in different frequency cells by amobile station UE according to this embodiment and a method oftransmitting a “Measurement Report” by the mobile station UE.

In Section A, the measurement unit 12 repeatedly measures the radioqualities in each measurement gap, in the order of the frequency cellF1, the frequency cell F2 and the frequency cell F3.

At a time point t1, the measurement report transmitter 13 determinesthat the radio quality in the frequency cell F2 (second frequency cell)satisfies any of the reporting criteria (for example, Event A5), andactivates the timer T2 (second timer corresponding to the aforementionedTtrigger) for the frequency cell F2.

In Section B, the measurement unit 12 stops measuring the radio qualityin the frequency cell F3 having the lower priority than the priority ofthe frequency cell F2, in the measurement gap, and starts measuring theradio quality in the frequency cell F2 in a period for measuring of theradio quality in the frequency cell F3.

To put it differently, in Section B, the measurement unit 12 repeatedlymeasures the radio qualities in the order of the frequency cell F1, thefrequency cell F2 and the frequency cell F2, in the measurement gap.

Subsequently, at a time point t2, if the timer T2 for the frequency cellF2 expires without being reset, the measurement report transmitter 13transmits a “Measurement Report” including the radio quality in thefrequency cell F2 to the radio base station eNB.

When “Event Triggered Periodical Reporting” is applied to the method oftransmitting a “Measurement Report,” the measurement report transmitter13 needs to transmit a “Measurement Report” N times at maximum atpredetermined reporting intervals (for example, every R1 second) evenafter a time point t3. Thus, in Section D, the measurement unit 12repeatedly measures the radio qualities in the order of the frequencycell F1, the frequency cell F2 and the frequency cell F2, in the entireperiod of the measurement gap.

Subsequently, at the time point t3, when determining that the radioquality in the frequency cell F1 (first frequency cell) having thehigher priority than the frequency cell F2 (second frequency cell)satisfies any of the reporting criteria (for example, Event A4), themeasurement report transmitter 13 activates the timer T1 (first timercorresponding to the aforementioned Ttrigger) for the frequency cell F1.

In section D, the measurement unit 12 starts measuring the radio qualityin the frequency cell F1 in a period for measuring of the radio qualityin the frequency cell F3, in the measurement gap.

Note that in a case where the measurement report transmitter 13transmits a “Measurement Report” including the radio quality in thefrequency cell F2 (second frequency cell) at predetermined intervals,the measurement unit 12 does not stop measuring the radio quality in thefrequency cell F2 (second frequency cell) in the period for measuringthe radio quality in the frequency cell F2 (second frequency cell).

Subsequently, at a time point t4, when the timer T1 for the frequencycell F1 expires without being reset, the measurement report transmitter13 transmits a “Measurement Report” including the radio quality in thefrequency cell F1 to the radio base station eNB.

The measurement report transmitter 13 needs to transmit the “MeasurementReport” N times at maximum at predetermined intervals even after thetime point t4. Thus, in Section E, the measurement unit 12 repeatedlymeasures the radio qualities in the order of the frequency cell F1, thefrequency cell F2 and the frequency cell F1, in the entire period of themeasurement gap.

In contrast, when “Event Triggered Reporting” is applied to the methodof transmitting a “Measurement Report,” the measurement reporttransmitter 13 transmits a “Measurement Report” at the time point t2,and thereafter in Section C, the measurement unit 12 resumes measuringthe radio qualities in the frequency cells F2 and F3 in periods formeasuring the radio qualities in the frequency cells F2 and F3.

Subsequently, at a time point t3, when determining that the radioquality in the frequency cell F1 (first frequency cell) having thehigher priority than the frequency cell F2 (second frequency cell)satisfies any of the reporting criteria (for example, Event A4), themeasurement report transmitter 13 activates the timer T1 (first timercorresponding to the aforementioned Ttrigger) for the frequency cell F1.

In Section D, the measurement unit 12 stops measuring the radioqualities in the frequency cells F2 and F3 each having the lowerpriority than the priority of the frequency cell F1, in the measurementgap, and starts measuring the radio quality in the frequency cell F1 inperiods respectively for measuring of the radio qualities in thefrequency cells F2 and F3.

To put it differently, in Section D, the measurement unit 12 measuresthe radio quality in the frequency cell F1 in the entire period of themeasurement gap.

Subsequently, at the time point t4, when the timer T1 for the frequencycell F1 expires without being reset, the measurement report transmitter13 transmits a “Measurement Report” including the radio quality in thefrequency cell F1 to the radio base station eNB. Then, in Section E, themeasurement unit 12 resumes measuring the radio qualities in thefrequency cells F2 and F3 in periods for measuring the radio qualitiesin the frequency cells F2 and F3.

MODIFICATION EXAMPLE

Note that the present invention is applicable to a case of Inter-RATmeasurement. In this case, it is possible to obtain the same effects asin the case of the aforementioned different frequency measurements.

Note that operation of the above described mobile station UE and theradio base station eNB may be implemented by means of hardware, asoftware module executed by a processor, or a combination of both.

The software module may be provided in any type of storage medium suchas an RAM (Random Access Memory), a flash memory, a ROM (Read OnlyMemory), an EPROM (Erasable Programmable ROM), an EEPROM (ElectronicallyErasable and Programmable ROM), a register, a hard disk, a removabledisk, or a CD-ROM.

The storage medium is connected to the processor so that the processorcan read and write information from and to the storage medium. Also, thestorage medium may be integrated into the processor. Also, the storagemedium and the processor may be provided in an ASIC. The ASIC may beprovided in the mobile station UE and the radio base station eNB. Also,the storage medium and the processor may be provided in the mobilestation UE and the radio base station eNB as a discrete component.

Hereinabove, the present invention has been described in detail usingthe above embodiment; however, it is apparent to those skilled in theart that the present invention is not limited to the embodimentdescribed herein. Modifications and variations of the present inventioncan be made without departing from the spirit and scope of the presentinvention defined by the description of the scope of claims. Thus, whatis described herein is for illustrative purpose, and has no intentionwhatsoever to limit the present invention. All content of the Japanesepatent application 2008-076896 (Filing date is 24 Mar. 2008) aredescribed into this application by the reference.

INDUSTRIAL APPLICABILITY

The present invention can provide a cell reselection method, a handovercontrol method, and a mobile station for which a specification isprovided to clarify, when a timer Treselection or a timer Ttrigger isactivated, how the mobile station UE should perform a cell reselectionor transmit a “Measurement Report” in multiple frequency cells assigneddifferent priorities.

The invention claimed is:
 1. A handover control method by which a radiobase station performs a handover control on a mobile station on a basisof a measurement report transmitted from the mobile station, the methodcomprising the steps of: measuring radio qualities in a plurality ofdifferent frequency cells by the mobile station in a predeterminedmeasurement period; when the mobile station determines that a radioquality in a specific frequency cell satisfies a reporting criterion,activating a timer by the mobile station, the timer being configured tomeasure a period during which the radio quality in the specificfrequency cell continues to satisfy the reporting criterion; and whenthe timer expires, transmitting the measurement report to the radio basestation by the mobile station, the measurement report including theradio quality in the specific frequency cell, wherein when the mobilestation determines that a radio quality in a first frequency cell havinga higher priority than a second frequency cell satisfies the reportingcriterion after determining that a radio quality in the second frequencycell satisfies the reporting criterion and activating a second timer,the mobile station stops the second timer and actives a first timer. 2.The handover control method according to claim 1, wherein when stoppingthe second timer and activating the first timer, the mobile stationstops measuring the radio quality in the second frequency cell andstarts measuring the radio quality in the first frequency cell, in aperiod for measuring the radio quality in the second frequency cell. 3.The handover control method according to claim 2, wherein aftertransmitting the measurement report including the radio quality in thefirst frequency cell, the mobile station resumes measuring the radioquality in the second frequency cell in the period for measuring theradio quality in the second frequency cell.
 4. The handover controlmethod according to claim 2, wherein even after transmitting themeasurement report including the radio quality in the first frequencycell, the mobile station continues measuring the radio quality in thefirst frequency cell in the period for measuring the radio quality inthe second frequency cell, and transmits the measurement reportincluding the radio quality in the first frequency cell at predeterminedintervals.
 5. The handover control method according to claim 2, whereinwhen transmitting the measurement report including the radio quality inthe second frequency cell at predetermined intervals, the mobile stationdoes not stop measuring the radio quality in the second frequency cellin the period for measuring the radio quality in the second frequencycell.
 6. A cell reselection method by which a mobile station reselects acell for performing a standby on a basis of radio qualities in aplurality of different frequency cells, the radio qualities beingmeasured in a predetermined measurement period, the method comprisingthe steps of: when the mobile station determines that a radio quality ina specific frequency cell satisfies a reselection criterion, activatinga timer by the mobile station, the timer being configured to measure aperiod during which the radio quality in the specific frequency cellcontinues to satisfy the reselection criterion; and starting the standbyin the specific frequency cell by the mobile station when the timerexpires, wherein when the mobile station determines that a radio qualityin a first frequency cell having a higher priority than a secondfrequency cell satisfies the reselection criterion after determiningthat a radio quality in the second frequency cell satisfies thereselection criterion and activating a second timer, the mobile stationstops the second timer and actives a first timer.
 7. The cellreselection method according to claim 6, wherein when stopping thesecond timer and activating the first timer, the mobile station stopsmeasuring the radio quality in the second frequency cell and startsmeasuring the radio quality in the first frequency cell, in a period formeasuring the radio quality in the second frequency cell.
 8. A mobilestation used in a mobile communication system configured such that aradio base station performs a handover control on the mobile station ona basis of a measurement report transmitted from the mobile station, themobile station comprising: a measurement device configured to measureradio qualities in a plurality of different-frequency cells in apredetermined measurement period; and a measurement report transmitterconfigured to activate a timer when the measurement report transmitterunit determines that a radio quality in a specific frequency cellsatisfies a reporting criterion, and to transmit the measurement reportto the radio base station when the timer expires, the timer beingconfigured to measure a period during which the radio quality in thespecific frequency cell continues to satisfy the reporting criterion,the measurement report including the radio quality in the specificfrequency cell, wherein when the measurement report transmitterdetermines that a radio quality in a first frequency cell having ahigher priority than a second frequency cell satisfies the reportingcriterion after determining that a radio quality in the second frequencycell satisfies the reporting criterion and activating a second timer,the measurement report transmitter stops the second timer and actives afirst timer.
 9. The mobile station according to claim 8, wherein whenstopping the second timer and activating the first timer, themeasurement report transmitter stops measuring the radio quality in thesecond frequency cell and starts measuring the radio quality in thefirst frequency cell, in a period for measuring the radio quality in thesecond frequency cell.
 10. The mobile station according to claim 9,wherein after the measurement report transmitter transmits themeasurement report including the radio quality in the first frequencycell, the measurement device resumes measuring the radio quality in thesecond frequency cell in the period for measuring the radio quality inthe second frequency cell.
 11. The mobile station according to claim 9,wherein even after the measurement report transmitter transmits themeasurement report including the radio quality in the first frequencycell, the measurement device continues measuring the radio quality inthe first frequency cell in the period for measuring the radio qualityin the second frequency cell, and the measurement report transmittertransmits the measurement report including the radio quality in thefirst frequency cell at predetermined intervals.
 12. The mobile stationaccording to claim 9, wherein when the measurement report transmittertransmits the measurement report including the radio quality in thesecond frequency cell at predetermined intervals, the measurement devicedoes not stop measuring the radio quality in the second frequency cellin the period for measuring the radio quality in the second frequencycell.
 13. A mobile station configured to reselect a cell for performinga standby on a basis of radio qualities in a plurality of differentfrequency cells, the radio qualities being measured in a predeterminedmeasurement period, the mobile station comprising: a measurement deviceconfigured to measure the radio qualities in the plurality ofdifferent-frequency cells in the predetermined measurement period; and acell reselector configured to activate a timer when the cell reselectordetermines that a radio quality in a specific frequency cell satisfies areselection criterion, and to start the standby in the specificfrequency cell when the timer expires, the timer being configured tomeasure a period during which the radio quality in the specificfrequency cell continues to satisfy the reselection criterion, whereinwhen the cell reselector determines that a radio quality in a firstfrequency cell having a higher priority than a second frequency cellsatisfies the reselection criterion after determining that a radioquality in the second frequency cell satisfies the reselection criterionand activating a second timer, the cell reselector stops the secondtimer and actives a first timer.
 14. The mobile station according toclaim 13, wherein when the cell reselector stops the second timer andactivates the first timer, the measurement device stops measuring theradio quality in the second frequency cell and starts measuring theradio quality in the first frequency cell, in a period for measuring theradio quality in the second frequency cell.